Make Redcarpet Markdown renderer thread-safe

The Redcarpet library is not thread-safe as described in https://github.com/vmg/redcarpet/issues/570. Since we instantiate the Redcarpet renderer in a class variable, multiple Sidekiq threads can access the work buffer and corrupt the state. We work around this issue by memoizing the renderer on a thread basis. Closes #36637

Make Redcarpet Markdown renderer thread-safe
15bebda7 · Stan Hu · 6c33fb84 · 15bebda7 · 15bebda7
Commit 15bebda7 authored Sep 30, 2017 by Stan Hu
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Showing with 19 additions and 18 deletions

sh-thread-safe-markdown.yml changelogs/unreleased/sh-thread-safe-markdown.yml +5 -0

markdown_filter.rb lib/banzai/filter/markdown_filter.rb +14 -18

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--- a/changelogs/unreleased/sh-thread-safe-markdown.yml
+++ b/changelogs/unreleased/sh-thread-safe-markdown.yml
+---
+title: Make Redcarpet Markdown renderer thread-safe
+merge_request:
+author:
+type: fixed
--- a/lib/banzai/filter/markdown_filter.rb
+++ b/lib/banzai/filter/markdown_filter.rb
 module Banzai
  module Filter
    class MarkdownFilter < HTML::Pipeline::TextFilter
+      # https://github.com/vmg/redcarpet#and-its-like-really-simple-to-use
+      REDCARPET_OPTIONS = {
+        fenced_code_blocks:  true,
+        footnotes:           true,
+        lax_spacing:         true,
+        no_intra_emphasis:   true,
+        space_after_headers: true,
+        strikethrough:       true,
+        superscript:         true,
+        tables:              true
+      }.freeze
+
      def initialize(text, context = nil, result = nil)
        super text, context, result
        @text = @text.delete "\r"
@@ -13,27 +25,11 @@ module Banzai
      end

      def self.renderer
-        @renderer ||= begin
+        Thread.current[:banzai_markdown_renderer] ||= begin
          renderer = Banzai::Renderer::HTML.new
-          Redcarpet::Markdown.new(renderer, redcarpet_options)
+          Redcarpet::Markdown.new(renderer, REDCARPET_OPTIONS)
        end
      end
-
-      def self.redcarpet_options
-        # https://github.com/vmg/redcarpet#and-its-like-really-simple-to-use
-        @redcarpet_options ||= {
-          fenced_code_blocks:  true,
-          footnotes:           true,
-          lax_spacing:         true,
-          no_intra_emphasis:   true,
-          space_after_headers: true,
-          strikethrough:       true,
-          superscript:         true,
-          tables:              true
-        }.freeze
-      end
-
-      private_class_method :redcarpet_options
    end
  end
 end